Entamoeba histolytica infection is extremely common and affects an estimated 480 million individuals annually. However, only about 10% of these persons develop symptoms such as colitis or liver abscess. The low incidence of symptom occurrence is putatively due to the existence of both pathogenic and nonpathogenic forms of the amoeba. As of 1988, it had been established that the subjects who eventually exhibit symptoms harbor pathogenic "zymodemes" which have been classified as such on the basis of their distinctive hexokinase and phosphoglucomutase isoenzymes. The pathogenic forms are not conveniently distinguishable from the nonpathogenic counterparts using morphogenic criteria, but there is an almost perfect correlation between infection with a pathogenic zymodeme and development of symptoms and between infection with a nonpathogenic zymodeme and failure to develop these symptoms.
It is known that E. histolytica infection is mediated at least in part by the "Gal/GalNAc" adherence lectin which was isolated from a pathogenic strain and purified 500 fold by Petri, W. A., et al., J Biol Chem (1989) 264:3007-3012. The purified "Gal/GalNAc" lectin was shown to have a nonreduced molecular weight of 260 kD on SDS-PAGE; after reduction with beta-mercaptoethanol, the lectin separated into two subunits of 170 and 35 kD MW. Further studies showed that antibodies directed to the 170 kD subunit were capable of blocking surface adhesion to test cells (Petri, et al. J Biol Chem (1989) supra). Therefore, the 170 kD subunit is believed to be of primary importance in meditating adhesion.
In addition, the 170 kD subunit is described as constituting an effective vaccine to prevent E. histolytica infection in U.S. Pat. No. 5,004,608 issued Apr. 2, 1991.
Studies of serological cross-reactivity among patients having symptomology characteristic of E. histolytica pathogenic infection, including liver abscess and colitis, showed that the adherence lectin was recognized by all sera tested (Petri, Jr., W. A., et al., Am J Med Sci (1989) 296:163-165). The lectin heavy subunit is almost universally recognized by immune sera and T-cells from patients with invasive amebiasis (Petri, et al., Infect Immun (1987) 55:2327-2331; Schain, et al., Infect Immun (1992) 60:2143-2146).
DNA encoding both the heavy (170 kD) and light (35 kD) subunits have been cloned. The heavy and light subunits are encoded by distinct mRNAs (Mann, B., et al., Proc Natl Acad Sci USA (1991) 88:3248-3252) and these subunits have different amino acid compositions and amino terminal sequences. The sequence of the cDNA encoding the 170 kD subunit suggests it to be an integral membrane protein with a large cysteine-rich extracellular domain and a short cytoplasmic tail (Mann, B., et al., Proc Natl Acad Sci USA (1991) supra; Tannich, et al., Proc Natl Acad Sci USA (1991) 88:1849-1853). The derived amino acid sequence of the 170 kD lectin shows that the extracellular domain can be divided into three regions on the basis of amino acid composition. The amino terminal amino acids 1-187 are relatively rich in cysteine (3.2%) and tryptophan (2.1%). Amino acid sequence at positions 188-378 does not contain cysteine, and the amino acid sequence at positions 379-1209 contains 10.8% cysteine residues. The obtention of clones encoding the heavy chain subunit is further described in U.S. Pat. No. 5,260,429 issued Nov. 9, 1993, the disclosure of which is incorporated herein by reference. In that patent, diagnostic methods for the presence of E. histolytica based on the polymerase chain reaction and the use of DNA probes is described.
The heavy subunit is considered to be encoded by a multigene family (Mann, B., et al., Parasit Today (1991) 1:173-176). Two different heavy subunit genes, hgl1 and hgl2, have been sequenced by separate laboratories. While hgl2 was isolated from an HM-1:IMSS CDNA library in its entirety (Tannich, E. et al. Proc Natl Acad Sci USA (1991) 88:1849-1853), hgl1 was isolated in part from an H-302:NIH cDNA library and in part by PCR amplification of the gene from the HM-1:IMSS genome (Mann, B. J. et al. Proc Natl Acad Sci USA (1991) 88:3248-3252). As the amino acid sequence of these two genes is 87.6% identical (Mann, B. J. et al. Parasit Today (1991) 7:173-176), the differences could be explained by strain variation alone. The presence of multiple bands hybridizing to an hgl probe on Southern blots, however, in consistent with the existence of a 170 kDa subunit gene family (Tannich, E. et al. Proc Natl Acad Sci USA (1991) 88:1849-1853).
Monoclonal antibodies specifically immunoreactive with various epitope-bearing regions of the 170 kD heavy chain subunit have also been disclosed in U.S. Pat. No. 5,272,058 issued Dec. 21, 1993, the disclosure of which is incorporated herein by reference in its entirety. This application also describes use of these antibodies to detect the 170 kD heavy chain and the use of the 170 kD subunit to detect antibodies in serum or other biological samples. The experimental work described utilizes the native protein. Further characterization of these antibodies is described in a publication by Mann, B. J., et al., Infect Immun (1993) 61:1772-1778 also incorporated herein by reference.
Various immunoassay techniques have been used to diagnose E. histolytica infection. ELISA techniques have been used to detect the presence or absence of E. histolytica antigens both in stool specimens and in sera, though these tests do not seem to distinguish between the pathogenic and nonpathogenic strains. In a seminal article, Root, et al., Arch Invest Med (Mex) (1978) 9: Supplement 1:203, described the use of ELISA techniques for the detection of amoebic antigen in stool specimens using rabbit polyclonal antiserum, and various forms of this procedure have been used, some in conjunction with microscopic studies. Palacios et al., Arch Invest Med (Mex) (1978) 9: Supplement 1:203; Randall et al., Trans Roy Soc Trop Med Hyg (1984) 78:593; Grundy, Trans Roy Soc Trop Med Hyg (1982) 76:396; Ungar, Am J Trop Med Hyg (1985) 34:465. These studies on stool specimens and on other biological fluids are summarized in Amebiasis: Human Infection by Entamoeba Histolytica, J. Ravdin, ed. (1988) Wiley Medical Publishing, pp. 646-648.
Conversely, amebic serology is also a critical component in the diagnosis of invasive amebiasis. One approach utilizes conventional serologic tests, such as the indirect hemagglutinin test. These tests are very sensitive but seropositivity is persistent for years (Krupp, I. M., Am J Trop Med Hyg (1970) 19:57-62; Lobel, H. O. et al., Ann Rev Microbiol (1978) 32:379-347). Thus, healthy subjects may give positive responses to the assay, creating an undesirable high background. Similar problems with false positives are found in using immunoassay tests involving a monoclonal antibody and purified native 170 kD protein (Ravdin, J. I., et al., J Infect Dis (1990) 162:768-772.)
Recombinant E. histolytica proteins other than the 170 kD subunit have been used as the basis for serological tests. Western blotting using a recombinant form of the "52 kD serine-rich protein" was highly specific for invasive disease and had a higher predictive value (92 vs. 65%) than an agar gel diffusion test for diagnosis of acute amebiasis (Stanley, Jr., S. L., et al., Proc Natl Acad Sci U.S.A. (1990) 87:4976-4980; Stanley, Jr., S. L., et al., JAMA (1991) 266:1984-1986). However, the overall sensitivity was lower than for the conventional agar gel test (82% vs. 90-100%).
Thus, there remains a need for serological tests which will provide optimum sensitivity while minimizing the number of false positives retained. The present invention provides such a test by utilizing, as antigen, epitope-bearing portions of the 170 kD subunit of the adherence lectin produced recombinantly in procaryotic systems.
It is particularly advantageous to use recombinantly produced, nonglycosylated peptides or proteins in this assay since these peptides are easily and efficiently obtained and are easily standardized. Furthermore, since selected portions of the lectin heavy chain subunit can be produced, epitopes characteristic of the pathogenic or nonpathogenic forms of E. histolytica can be produced and used to distinguish these forms in the assays. Subsequent to the invention herein, a report of immunoreactivity of recombinant 170 kd lectin with immune sera was published by Zhang, Y, et al. J. Clin Micro-immunol (1992) 2788-2792. Applicants incorporate by reference their own publication: Mann, B. J et al. Infect and Immun (1993) 61: 1772-1778.
Similarly, although it is known that the 170 kD subunit may be used as a vaccine as described in the above-referenced U.S. Pat. No. 5,004,608, recombinantly produced forms of the 170 kD subunit, specifically those obtained from procaryotic cells that lack glycosylation may offer advantages in reproducibility of product and in ease of preparation of subunit vaccines. The present invention is directed to this desirable result.